1. Field of the Invention
The present invention relates generally to glass sealing frits such as alkali-zinc-silicate frits. More specifically, these frits are suitable as sealing frits for solid oxide fuel cells (SOFC).
2. Technical Background
Frits which seal in the temperature range of 600° C. to 1000° C. represent an intermediate class of materials between the B2O3 or P2O5 based frits used for low temperature sealing of many commercial glass products and the diverse number of silicates utilized for high temperature joining of advanced ceramic, structural components.
The low temperature frits are used at temperatures below 600° C. for sealing products such as cathode ray tubes (CRT), light bulbs and the like. The high temperature frits are used at temperatures above 1000° C. to produce articles which may embody high temperature, fiber reinforced, structural ceramics.
A very old product in the intermediate temperature range (600° C. to 1000° C.) of sealing is ZnO—B2O3—SiO2 frit. Another is Li2O-modified ZnO—Al2O3—SiO2 frit designed for use between 900° C. to 1000° C. Frits that seal in the range between 600° C. and 900° C. are important for many applications. The need for such frits became evident through requests for sealant frits for solid Oxide fuel cells (SOFC).
Furthermore, fuel cell devices undergo large thermal cycling and large thermal gradients, which induces thermal stresses in the fuel cell stack components. Thus the seals need to be able to withstand high temperature fluctuations and have expansion coefficients compatible with electrolyte sheets and frames. If the seal will expand at a rate that is different from the thermal expansion rate of the frame or the electrolyte sheet, it may either crack or cause cracking of the electrolyte sheet. A defect in either the seal or the electrolyte sheet may necessitate a replacement of the electrolyte device.
U.S. Pat. No. 6,291,092 describes the B2O3-free-frit seals suitable for use in solid oxide. These frit seals had been successfully used in many applications. However, some of the fuel cell devices utilize steel components (e.g., frames or substrates). The frit seals disclosed in Table 1 of U.S. Pat. No. 6,291,092 contain relatively high amounts of ZnO, a compound that could be reduced under appropriate conditions by redox reaction with many stainless steel alloying elements such as Cr and Al.
Thus the need to have alternative frit seal compounds for solid oxide fuel cells has been the subject of considerable amount of research in recent years.